Curd is Indian term and also called Dahi. It is made by boiling and cooling the milk to ~32°C and adding a spoonful of curd. Now curd has lactic acid bacteria or lactobacillus. This bacteria multiplies itself in the ambient temperature of 30-40°C and in few hours ferments the milk to form curd. Curd is a rich source of calcium, protein and is also suitable for lactose intolerant people. It helps in digestion and gives a cooling relief from spicy foods. But because curd and its bacterial strength vary from one home to the other, it is not a standardised product.
Yogurt is prepared using similar techniques to curd but the fermentation of the milk is done by adding two specific strains of bacteria called lactobacillus bulgaris and streptococcus thermophilus. Other strains of lactic acid bacteria may also be added. The addition of these bacteria makes the product standardised and homogenous. This ensures both quality and right quantity of bacteria in the yogurt.